1. Field of the Invention
The present invention relates to a method for controlling a variable displacement type refrigerant compressor employed for compressing a refrigerant of a car air-conditioner in relation to the closed circuit operation of the car air-conditioner having a closed refrigerant circuit incorporating therein a refrigerant condensor, an expansion valve, and an evaporator. More particularly, the invention relates to a method for controlling the operation of a displacement changing mechanism of a wobble plate type variable displacement compressor to prevent hunting of a controlled variable of an air-conditioner, which occurs when interference occurs between the controlled variable of the air-conditioner and a manipulated variable of a control means used for causing a change in the controlled variable by controlling the operation of the compressor, and to achieve the optimum operation of the variable displacement type compressor.
2. Description of the Related Art
Recent car air-conditioners are employing a variable displacement compressor for compressing a refrigerant gas medium, e.g., a variable displacement wobble plate type compressor, as disclosed in the allowed U.S. patent application Ser. No. 093,797 for the same Applicants. The variable displacement wobble plate type compressor is provided with a displacement changing mechanism to vary the compressor displacement, i.e., the delivery capacity of the compressor. For example, the displacement changing mechanism of the variable displacement wobble plate type compressor of the allowed U.S. patent application Ser. No. 093,797 comprises a solenoid valve means for changing the angularity of a wobble plate assembly in response to a requirement of the car air-conditioner, thereby increasing or decreasing the stroke of the pistons of the compressor.
As shown in FIG. 7, in the conventional method for controlling the operation of the variable displacement wobble plate type compressor especially used for a car air-conditioner having an evaporator as a heat exchanger, a desired value corresponding to a desired temperature in the compartment of a car is initially set at the start of the operation of the air-conditioner. Then, detection of the temperature of the outlet of the evaporator or that in the car compartment is carried out to detect a cooling load of the air-conditioner. Subsequently, calculation of a manipulated variable is carried out by an operating circuit for calculating the manipulated variable based on the desired temperature and the detected temperature at the outlet of the evaporator or in the car compartment to operate the displacement changing mechanism of the compressor. Accordingly, the operation of the variable displacement type refrigerant compressor is adjusted so that a controlled variable of the air-conditioner reaches an appropriate value.
Nevertheless, since the car air-conditioner has a closed refrigerant circuit in which the compressor, a refrigerant condenser, an expansion valve, and an evaporator are arranged and serially connected to one another, an adjustment of the displacement of the compressor carried out by the manipulated operation of the displacement changing mechanism of the compressor on the basis of the set desired temperature and the detected temperature at the evaporator outlet cannot cause an immediate reaction by the evaporator of the air-conditioner to regulate a controlled variable of the air-conditioner. Namely, a delay or a time lag appears between the manipulated operation of the compressor and the corresponding change in the controlled variable of the air-conditioner, and therefore, a hunting of the controlled variable oscillating up and down with respect to the desired temperature value occurs, and accordingly, a temperature at the outlet of the evaporator is greatly changed, which gives a passenger in the car compartment an uncomfortable feeling.